UNIT 5

 

I. Find out the words in the dictionary. Write them down and learn.

 

strength, elasticity, stiff, to stretch, to compress, strain, tensile, shear, compressive, to twist out, to pull apart, repulsive, bond, hardness, pressure, density, to expand, cohesion, tension, to cancel out

 

II. Read the text. Use a dictionary, if necessary.

 

Text: “Properties of Solids and Liquids”.

 

The forces that hold atoms and molecules in place give solids their strength.

The strength of a material is determined by measuring its elasticity – that is how stiff it is and how it behaves when it is stretched or compressed. Elasticity is measured as the ratio of stress to strain.

Stress is defined as the force acting on a material divided by the area over which the force is applied. Stress may be applied in three different ways: tensile, shear and compressive.

Tensile stress causes a material to become elongated (stretched), and shear stress causes it to be twisted out of shape. In both cases the component atoms or molecules are pulled apart as they move from their original positions. When the stress is removed, the attractive forces between the atoms or molecules pull them back together again, and the material is restored to its original shape. Compressive stress forces the atoms or molecules together, and it is the repulsive forces that restore the shape of the material when the stress is removed. If stress is applied to a material and slowly increased, the material accordingly changes shape. But at a certain point it reaches its elastic limit.

Another property of solid materials that depends on the strength of the bonds between atoms or molecules is hardness.

As in a solid, the atoms or molecules of a liquid are held together by attractive forces. But these forces are not great enough to hold the atoms or molecules in a fixed pattern, they move about on random. As a result, a liquid can flow and it cannot be stretched or distored. Like a solid, it can be compressed slightly and shows the same sort of elasticity when subjected to compressive stress. Unlike a solid, but like a gas, a liquid exerts pressure, which at any point depends on the depth and the density of the liquid.

A liquid does not expand to fill the whole of the volume available to it. Instead, the cohesion between its molecules forces it to maintain a fixed volume (at a given temperature). It takes on the shape of all or part of its container – but this is only because the forces of gravity makes it do so. In zero gravity conditions a liquid takes on the shape with the minimum possible surface area – that is, a sphere.

The fact that a liquid tends to take on a spherical shape is due to a phenomenon known as surface tension. Inside a volume of liquid all the atoms or molecules attract each other equally. Each molecule experiences attractive forces in all directions and they cancel out each other. But the atoms or molecules on the surface experience few, if any, attractive forces from the outside. As a result, the forces between them and inner molecules tend to pull them inwards and towards each other.

 

III. Define by suffix speech parts of the following words and translate them:

 

Generally, collection, elasticity, define, different, compressive, position, attractive, repulsive, restore, slowly, remove, accordingly, hardness, move, density, available, container, spherical, phenomenon, equally, direction, inner.

 

IV. Find English equivalents to the following expressions.

 

1. сила _______________________________________________________
2. прочность __________________________________________________
3. жесткий ____________________________________________________
4. нажим/давление _____________________________________________
5. быть искривленным _________________________________________
6. сила притяжения ____________________________________________
7. отталкивающая сила _________________________________________
8. соответственно изменять форму _______________________________
9. точка сопротивления _________________________________________
10. связи между атомами или молекулами _________________________
11. наугад _____________________________________________________
12. оказывать давление __________________________________________
13. расширяться ________________________________________________
14. сцепление/соединение ________________________________________
15. сила тяжести ________________________________________________
16. поверхность _________________________________________________
17. напряжение _________________________________________________

 

V. Find Russian equivalents to the following words and expressions:

 

1. to hold atoms and molecules ____________________________________
2. the strength of a material _______________________________________
3. stiff ________________________________________________________
4. may be applied _______________________________________________
5. to become elongated ___________________________________________
6. molecules are pulled apart ______________________________________
7. attractive force _______________________________________________
8. to pull back __________________________________________________
9. repulsive forces ______________________________________________
10. to restore ___________________________________________________
11. a fixed pattern _______________________________________________
12. the density of the liquid ________________________________________
13. surface tension _______________________________________________
14. to pull inwards and towards _____________________________________ 

 

VI. Fill in the gaps with the following words.

 

tension, to strain, tensile, liquid, compressive, bonds, strength, hardness, ratio, shear, repulsive, to stretch, exerts, solid

 

1. The forces that hold atoms and molecules in place give solids their ____.
2. Elasticity is measured as the ____ of stress to ____.
3. Stress may be applied in three different ways ____, _____, _____.
4. ____ stress causes a material to become elongated.
5. ____ stress forces the atoms or molecules together, and it is the ____ forces that restore the shape of the material.
6. Another property of ____ materials that depends on the strength of the ____ between atoms and molecules is ____.
7. A ____ can flow.
8. A liquid ____ pressure.
9. The fact that a liquid tends to take on a spherical shape is due to a phenomenon known as surface ____.

 

VII. Fill in the prepositions, if necessary.

 

1. The strength ____ a material is determined ____ measuring its elasticity.
2. Shear stress causes it to be twisted _____ shape.
3. In both cases the component atoms or molecules are pulled ___ as they move from their original positions.
4. The material accordingly changes _____ shape.
5. Another property of solid materials that depends _____ the strength of the bonds _____ atoms and molecules is hardness.
6. Each molecule experiences attractive forces _____ all directions.
7. ____ a volume of liquid all the atoms or molecules attract each other equally.

 

VIII. Define whether the statements are true or false.

 

1. The forces that fold atoms and molecules in place don’t give solids their strength.
2. The strength of a material is determined by measuring its elasticity.
3. Stress is defined as the force acting in a material.
4. Stress may be applied in four different ways.
5. Another property of solid materials that depends on the strength of the bonds between atoms and molecules is softness.
6. Atoms or molecules of a liquid are held together by attractive forces.
7. These forces in liquids are great enough to hold atoms or molecules in a fixed pattern.
8. A liquid always expands to fill the whole of volume available to it.
9. The fact that a liquid tends to take on a spherical shape is due to surface tension. 

 

IX. Put questions to the following statements.

 

1. The forces that hold atoms and molecules in place give solids their strength.
2. The strength of a material is determined by measuring its elasticity.
3. Elasticity is measured as the ration of stress to strain.
4. The atoms or molecules of a liquid are held together by attractive forces.
5. A liquid can flow.
6. A falling drop of water tends to take the shape of a sphere.
7. Another property of liquids is adhesion.

 

X. Dictation – translation.

 

1. Силы, которые удерживают атомы и молекулы на месте, дают твердым телам прочность.
2. Прочность материала определяется измерением его эластичности.
3. Эластичность измеряется как отношение прочности к растяжению.
4. Напряжение – это сила, которая воздействует на материал.
5. Если давление на материал увеличить, то материал изменит свою форму.
6. Атомы и молекулы удерживаются силой притяжения.
7. Жидкость оказывает давление, которое в любой точке зависит от глубины и плотности жидкости.
8. Жидкость пытается принять сферическую форму.
9. Все атомы и молекулы в жидкости притягивают друг друга с одинаковой силой.